Petrogenetic significance of Au–Bi–Te–S associations: The example of Maldon, Central Victorian gold province, Australia

Lithos 116 (2010) 1–17

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Petrogenetic significance of Au–Bi–Te–S associations: The example of Maldon,Central Victorian gold province, Australia

Cristiana L. Ciobanu a,b,⁎, William D. Birch c, Nigel J. Cook a,b, Allan Pring a,b, Pascal V. Grundler a,b

a South Australian Museum, Adelaide, S.A., Australiab School of Earth and Environmental Sciences, University of Adelaide, S.A., Australiac Museum Victoria, Melbourne, Victoria, Australia

⁎ Corresponding author. School of Earth and EnvironAdelaide, S.A., Australia. Tel.: +61 405 826057.

E-mail address: [email protected] (

0024-4937/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.lithos.2009.12.004

a b s t r a c t
a r t i c l e i n f o
Article history:Received 17 September 2009Accepted 16 December 2009Available online 4 January 2010

Keywords:Gold depositsBismuth meltsFocussed ion beamTransmission electron microscopyMaldoniteTetradymite groupRemobilisationThermodynamic modelling

Mineralization at Maldon, Victorian goldfields, Australia, is part of the Western Lachlan Foldbelt and is hosted bydeformed Lower Ordovician metasediments intruded by the Late Devonian Harcourt Granite. Gold–Bi–Te–Sassociations in the deposit include four gold-bearing minerals (maldonite, native gold, jonassonite, and aurostibite)and a range of sulphotellurides from the tetradymite group, all of which are paragenetically tied to native bismuth.The simpler phase associations, involving bismuth, maldonite and one or the other Bi-(sulpho)tellurides, resemblethe equivalents of eutectics in the system Au–Bi–Te and hint at crystallisation frommelts. The complex associationsinclude symplectitesof goldandbismuth resulting fromreplacementofmaldonite andaredominatedby co-existing,sulphur-bearing species (joséite-A, joséite-B, an unnamed phasewith bulk composition approximating to Bi3(Te,S)2and jonassonite). The complexity of such assemblages is also underpinned by chemical–structural inhomogeneitywithin the unnamed phase, i.e. nanoscale domains of Bi14(Te,S)9 and Bi5(Te,S)3, as documented from electrondiffractions obtained from a FIB-TEM foil. Although such lattice-scale intergrowths reveal polysomatic disorder thatcan be associated with replacement of one telluride by another, the structural modules in the overall stackingsequence indicate formation under local equilibrium conditions.The mineralogical complexity of all Au–Bi–Te–S associations can be modelled in terms of interaction betweensimpler assemblages and sulphur-bearing fluids. Three stages—all involving gold minerals—are recognised:(1) bismuth+maldonite±hedleyite; (2) Bi-sulphotellurides+jonassonite, and decomposition of maldonite(gold+bismuth); and (3) bismuthinite+gold from decomposition of maldonite or jonassonite. The last stage iscoincidentwith chloritisation andoccursduringor followinggranite emplacement. Thermodynamicmodelling ofmaldonite replacement by bismuthinite and gold indicates conditions to overlapwith the pyrite–hematite bufferat 258 °C if neutral fluids are involved. The general replacement model for the deposit shows that local-scalereworking of maldonite and resultant gold remobilisation contributed to the apparent abundance of native gold.This interpretation is consistent with the protracted geological history at Maldon where granite intrusion post-dates an orogenic event. Initial sulphur-poor assemblages crystallised from melts predating graniteemplacement. The latest sulphidation event is attributed to retrograde fluids in the contact aureole, whereasthe earlier one could also have been produced during the multi-stage deformation. Although observedassemblages and textures are the product of a complex sequence of overprinting, there is noneed to invoke inputof gold from more than a single fluid generation.

mental Sciences, University of

C.L. Ciobanu).

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Although the widespread association between gold mineralisationand bismuth minerals has long attracted attention and has beeninterpreted in genetic terms (e.g. Spooner 1993; Marcoux et al., 1996;Meinert 2000), it is only recently that such associations have beenconsidered pivotal for understanding processes of gold deposition andremobilisation. One of the keyminerals within Bi-mineral associations

in gold deposits is maldonite (Au2Bi), the only naturally occurringcompound of the two elements, named after the type locality in theVictorian goldfields, Australia (Ulrich, 1869). The history of maldoniteand aspects of maldonite mineralogy from the type locality arediscussed by Birch and Ciobanu (2009).

Paragenetic, as well as temporal/spatial relationships between goldand bismuth minerals mirror a shared response to the physical andchemical ore-forming environment. This understanding is achieved viadetailed investigations of different occurrences, with emphasis on theinterpretation of ore textures, recognition of phase equilibria in naturalsamples using relationships in the system Au–Bi–Te, element partition-ing between co-existing phases, and assessing the role that modular

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structures may play in defining equilibrium in observed assemblages(Ciobanu et al., 2006a; Cook et al., 2009a). The recently publishedresearch summarised below shows why gold and bismuth mineralassemblages can be used to model formation of some gold deposits.

1.1. Melt scavengers for gold

Interpretation of textures and phase relationships observed in oreswhere sulphosalts and tellurides are minor components has led to therelatively new concept of partial melting as a viable mechanism toconcentrate metals since these minor minerals are composed of oneor more low melting point chalcophile elements (LMCE; Frost et al.,2002). More important to the topic here is the role such melts canhave in scavenging gold, with excellent application to the Au–Bi–Te–(S, Se) mineral systems (Ciobanu et al., 2005). Gold–Bi–Te melts canincorporate as much as 43 wt.% Au (the amount of Au for the eutecticat 447 °C in the Au–Bi–Te system; Prince et al., 1990) and will thus actas scavengers for Au if the opportunity to form melts arises. Theeffectiveness of Bi-melt scavenging of gold, even from fluids notsaturated in this element, has been shown both experimentally(Douglas et al., 2000) and by thermodynamic modelling (Wagner,2007; Tooth et al., 2008).

Nativebismuth (meltingpoint271 °C)andAu–Bi–Te–(S) assemblagesare molten at temperatures that overlap the formation conditions of alarge range of gold deposits. A model involving Au–Bi–Te–(S) meltsprecipitated from hydrothermal fluids has been considered, based ontextural and phase relationships, for interpretation of various deposittypes. These include intrusion-related gold (Pogo and Fort Knox, TintinaBelt, Alaska; McCoy, 2000), epithermal–porphyry systems (Larga, GoldenQuadrilateral, Romania; CookandCiobanu, 2004), recentvolcanicmassivesulphide systems (Escanaba Trough, S Gorda Ridge; Törmänen and Koski,2005), skarn deposits such as Fe skarns (Ocna de Fier and Baisoara,Romania; Ciobanu et al., 2003; Ciobanu and Cook, 2004) and Au skarns(Ortosa and El Valle, Rio Narcea Gold Belt, Spain; Cepedal et al., 2006), aswell as fororogenicgold systems (ViceroyMine,Harare–Bindura–Shamvagreenstonebelt, Zimbabwe;OberthürandWeiser, 2008).Moreover,meltscan scavenge Au from a pre-existing ore if partial melting occurs duringdeformation associated with high-grade metamorphism (e.g. Tomkinset al., 2007). In thecaseofBi-dominantmelts, this canbe initiatedas lowasthe upper greenschist facies (400 °C), e.g. the estimated conditions forAlpine Au–Bi–S remobilizates from the Highis Massif, Romania (Ciobanuet al., 2006b).

The presence of LMCEs as minor/trace components in gold carrierscan also assist remobilisation of Au during protracted geological events,as recently shown for pyrite ores in deposits from the North ChinaCraton (Cook et al., 2009b). In this case, LMCEs such as Te, Pb, Bi, Ag andCu, will scavenge gold during the interaction between pyrite and fluids.Although Bi or Te, alone, are efficient scavengers, the scavenging effectof the other mentioned elements is only significant when combinedwith one another in a LMCE association. Gold enrichment in the As-freepyrite from these deposits can be as high as several thousand ppm.

1.2. Bismuth tellurides: insights into phase equilibria using chemical–structural modularity

Bismuth tellurides (selenides, tellurosulphides and tellurosele-nides) forming both the homologous tetradymite group (Cook et al.,2007a and references therein; see also Moëlo et al., 2008) and the‘aleksite’ series (Cook et al., 2007b) are not only prominent accessorycomponents in many gold deposits but can also themselves be goldcarriers (Ciobanu et al., 2009a). Minerals from both series are mixed-layer compounds with rhombohedral or trigonal symmetry, derivedfrom the same 5-layer module (X–Bi–X–Bi–X; X=Te, Se, S), knownas the ‘tetradymite archetype’, by incremental addition of Bi–Bi andM–X (M=Pb, Bi), respectively (Cook et al., 2007a,b; Ciobanu et al.,2009b). Bismuthinite, Bi2S3, which has orthorhombic symmetry,

is not part of the tetradymite group. Each stoichiometric incrementwithin the tetradymite group (e.g. Bi2X3, Bi4X3) is represented byan isoseries (e.g. tellurobismuthite Bi2Te3, tetradymite Bi2Te2S,kawazulite Bi2Te2Se; or pilsenite Bi4Te3, joséite-B Bi4Te2S, joséite-ABi4TeS2, ikunolite Bi4SeS2—also given as Bi4S3 since natural ikunolitemay be Se-free).

Using high-resolution transmission electron microscopy (HR-TEM)to investigate compounds in the compositional range Bi2Te3–Bi8Te3,Ciobanu et al. (2009b) have shown that all phases are N-fold (N=totalnumber of layers in the unit cell) superstructures of a rhombohedralsubcell and that homology is underpinned by the structural formula S′(Bi2kX3)·L′(Bi2(k+1)X3), where X=chalcogen and S′ and L′ are thenumber of short and longmodules, respectively. Based on this, the studyintroduces amethod to calculate and evaluate stacking sequences usinga computing program and electron diffraction patterns, respectively.Moreover, the structural factor k is important to discriminate thosephases formed at equilibrium (Ciobanu et al., 2009b). In a multi-phaseassemblage, the k factor in the formulae of the component mineralsmust be consecutive to be formed at equilibrium (e.g. joséite-B andhedleyite, where k=2 and 3, respectively, can co-exist at equilibrium,whereas tetradymite (k=1) and hedleyite (k=3) cannot). On thephase diagrams for the systems Bi–Te and Bi–Se (Okamoto and Tanner,1990; Okamoto, 1994), any infinitesimal change in composition can bestabilised by modifications to the stacking sequence. According to theformula above and theunderlying theory, and confirmedby theelectrondiffractions of Ciobanu et al. (2009b), such phases can only be built bymodules with consecutive k.

Basedonour empirical observations on a largenumber of occurrences,the Bi:Te(+S, Se) ratio of phases in the tetradymite group can be used tocharacterise the type of mineral association as either reduced or oxidised.This carries implications for the stability of the associated gold minerals(native gold, maldonite, jonassonite and gold tellurides) as well as othercomponents such as native bismuth or tellurium (e.g. Ciobanu et al.,2005). Phases with Bi:Te(+S, Se)N1 are typical of reduced associationsand co-exist with native bismuth, maldonite, jonassonite; pyrrhotite andmagnetite are the stable ironminerals. In contrast, phaseswith Bi:Te(+S,Se)b1 are found in oxidised associations co-existingwithnative telluriumand gold tellurides; this coincides with the stability fields of pyrite andhematite.

In this study, the crystal-chemistry and phase relationships of gold–bismuth–telluride assemblages from the type locality for maldonite, agood example of a reduced assemblage, are analysed with the purposeof providing direct insight into the genesis of gold mineralisation. Theresults carry implications for modelling deposits with comparablemineral signatures in other locations worldwide.

2. The Maldon deposit

Gold was first found in the Maldon district (then known asTarrengower), about 135 km northwest of Melbourne, in 1854. Maldonis one of themedium-sized deposits within the central Bendigo Zone ofthe Central Victorian gold province (Philips and Hughes, 1998; Bierleinet al., 2001; Fig. 1). Over twenty quartz reefs were worked along a N–S-trendingbelt some6 km long and 3 kmwide. Themajor producing reefswere theNuggetty, at the northern edge of the field; the Eaglehawk andLinscott line; the Beehive and South German line, and the Parkins reef,containing the North British Mine, at the southern end of the field(Fig. 2). By 1926, about 1.8 Moz (56 t) of gold had been won, at anaverage grade of 28 g/t. Maldonite has been mentioned from severalreefs, but was found with most certainty from the Nuggetty reef and atthe North British Mine.

Mineralisation in the Maldon goldfield is hosted by quartz reefsoccupying fracture systems cutting Lower Ordovician metasedimentsof theWestern Lachlan Foldbelt of Eastern Australia (Mason andWebb,1953; Cherry andWilkinson, 1994). Severalmajor deformational eventshave affected the goldfield, giving rise to a variety of fault systems

Fig. 1. Regional geological map of the Central Victorian gold province (CVGP) modified after Bierlein et al. (2004), showing the distribution of the main gold deposits. Gold deposittonnages represent production totals (primary+placer) simplified and updated from Philips and Hughes (1998). At Maldon, only 14% of old production was taken from placers.Inset shows the location of the CVGP within the Western Lachlan Orogen.

3C.L. Ciobanu et al. / Lithos 116 (2010) 1–17

and reef structures with associated alteration (Ebsworth et al., 1998).The Late Devonian Harcourt Granite intruded the isoclinally-foldedsequence. Most of the exploited reef systems are placed within theK-feldspar zone of the contact aureole (Gregory, 1994; Fig. 2). Peakmetamorphism in the contact aureole is given as 1–2 kbar and≥500 °C (Hack et al., 1998). Intrusive dykes, pegmatitic veins andhydrothermal mica and altered wallrock at Maldon give ages rangingfrom 365±2 to 373±2 Ma (Bierlein et al., 2001).

Fig. 2. Geological sketch map of the Maldon goldfield after Bierlein et al. (2001) showingthe main gold–quartz reefs mentioned in the text and the position of North British Minewhere some of the samples in this study are located. Isograds and occurrence of key oreminerals in the orefield are after Hughes et al. (1997); a = arsenopyrite, l = löllingite+pyrrhotite+arsenopyrite, m = maldonite, p = pyrite.

The reef systems are mainly within high-angle fractures associatedwith the overturned western limbs of some anticlines, which strikeslightly west of north, with steep east-dipping axial planes. The systemsare open to the south but truncated in the north by the Harcourt Granite.The relationship between quartz reefs and fault systems has beendiscussed by Moon (1897), Caldwell et al. (1926), Hack et al. (1998)and Ebsworth and Krowkowski de Vickerod (1998). Gold mineralisationoccurs in pipe-like shoots and in small cross-cutting fractures. The reefsdisplay a complex history of vein formation, faulting and alteration; gold-bearing reefs have been attributed to D3–D6 structures (Ebsworth et al.,1998).

Unlike in other deposits in the Victorian Goldfields, in which gold–pyrite–arsenopyrite is the dominant assemblage (Hughes et al., 1997;Phillips et al., 2003), ores at Maldon are rarely sulphide-rich. A range ofsulphides and related minerals has, however, been reported in variousstudies (e.g. Hack et al., 1998; Ebsworth and Krowkowski de Vickerod,1998; Ebsworth et al., 1998). As well as pyrite and arsenopyrite, bothlöllingite and pyrrhotite are noted components of the Maldon ore(Fig. 2), which has an atypically high pyrrhotite to pyrite ratio(Ebsworth et al., 1998 and references therein). The telluride speciestetradymite and joséite had been reported from Maldon; Birch (1979)identified joséite-A, Ebsworth et al. (1998) mentioned unnamed Bi3Te(Se,S) and Ebsworth and Krowkowski de Vickerod (1998) referred toother unspecified unnamed tellurides.

The various sulphide and telluride assemblages have been attributedto several stages of reef formation and alteration, with the overridingdiscussion centred on the origin and timing of the Au–Bi–Te mineralisa-tion, in particular whether it was introduced or remobilised by theintrusion of the Harcourt Granite (e.g. Gregory 1994; Ebsworth andKrowkowski de Vickerod, 1998). Reactivation of D4 structures relating togranite emplacement is considered to have implications for substantialchanges in mineralogy, as for example, replacement relationshipsbetween arsenopyrite, pyrrhotite, löllingite and pyrite (see Hughes etal., 1997; Ebsworth and Krowkowski de Vickerod, 1998; Ebsworth et al.,1998). A prevailing opinion (e.g. Hughes et al., 1996) is that thewidespread granites intruding the Victorian gold province were neithera source of the gold-bearing fluids nor did they play a critical role in oregenesis. They are, however, believed to have contributed to the localenrichment in bismuth and formation of maldonite. Bierlein et al. (2004)


pointed out that the Maldon deposit exhibits characteristics of orogeniclode mineralisation overprinted by an intrusive-related gold system andthat these overprinting relationships may have led to the complexity ofthe mineral assemblage in the deposit.

3. Ore petrology

Our study is a detailed petrological investigation and geochemicalmodelling of high-grade Au–Bi ore. The samples studied are fromNorth British Mine from the collection of Museum Victoria, and alsomaterial from an unspecified locality at Maldon. The latter wereinvestigated by Paul Ramdohr in 1949 during his visit to Melbourneand donated to the museum (Table 1). Transmission electron micro-scopy (TEM) is used to characterise lattice-scale homogeneity in one ofthe phases from the tetradymite group and determine its implicationsfor petrogenesis.

The samples are ideally suited to provide additional constraints onthe importance of bismuth mineralogy in understanding gold ores.Previous viewpoints on genetic aspects at Maldon are considered inthe light of evidence obtained in this study and discussed later inthis paper.

3.1. Experimental

Polished blocks prepared from the samples were analysed byscanning electron microscopy (SEM), electron microprobe (EPMA)and TEM using facilities at the Adelaide Microscopy Centre, Universityof Adelaide, Australia.

EPMA data were collected using a Cameca SX-51 instrument.Operating conditions were an accelerating voltage of 20 kV and a beamcurrent of 20 nA. Standardsusedwere:Au (Au), Bi2Se3 (Bi, Se), PbS (Pb, S),Ag2Te (Ag, Te), Sb2S3 (Sb) and both CoAsS and GaAs (As). A Philips200CM transmission electron microscope was used for electron diffrac-tion. Sample preparation for TEM was done by Focused Ion Beam (FIB)method (Wirth, 2009), using a FEI Helios nanoLab DualBeam FIB/SEMsystem.

3.2. The Au–Bi–Te–S associations

Phases from the tetradymite group are present in spatial associationwith native gold or Au-bearing minerals (speciation/distribution andcomposition for allminerals are given in Tables 1 and 2–4, respectively).Whereas native bismuth is ubiquitous, although in variable amountswithin any of the individual associations, common sulphides are veryrare in the samples, if present at all. The main ganguemineral is quartz.

The Au–Bi–Te–S assemblages occur as larger patches or small blebs(frommm- to b10 µm in size, respectively) and thin veinlets in ‘seamy’quartz (Fig. 3a, b). Chlorite and bismuthinite are seen together inpressure gashes along some of the veinlets (Fig. 3c). In some cases the

Table 1Samples used in the study and their mineralogy.

Sample (polished block) ID Location Nativeelements

Au-bearing minerals

Bi Au Mld Jon

Au2Bi AuBi5S4

*M49328 (m1) Unknown X X X*M49328 (m2) Unknown X x xM49251 (m3) N. British Mine X X x xM42405 (m4) N. British Mine X X xM42405 (m5) N. British Mine X X x x

*Refers to specimens collected by Paul Ramdohr during his visit to Melbourne in 1949 in thM49328/Mld2 is the sample containing the blebs discussed in the text.Abbreviations: Mld—maldonite, Jon—jonassonite, Asb—aurostibite, Hed—hedleyite, JoB—joséite—B

Au–Bi–Te–Sminerals cementbrecciated quartz or are embeddedwithinclots of K-feldspar—(biotite) hornfels showing chloritic alteration(Fig. 3d). The altered fragments may also include variable amountsof apatite, barite, ankerite or rutile. Bismuthinite, however, more oftenenvelopes the Au–Bi minerals, or occurs at replacement boundariesbetween chlorite and the Au–Bi–Te–S minerals (Fig. 3d inset, e). Suchtextures clearly indicate its association with late alteration. Althoughwidespread, bismuthinite is present in minor quantities relative to theother components of the Au–Bi–Te–S patches. One of the peculiarities isthat it has a variable content of Sb (up to 8.91wt.%; Table 3). The Sb-richzones are irregular, with sheared or patchy appearance. Bismuthinite–aikinite compositions are commonly buffered by native bismuth in theabsence of sulphur (Pring, 1995).

3.3. Gold minerals

There are four gold minerals present at Maldon: native gold,maldonite, aurostibite and jonassonite [Au(Bi,Pb)5S4; Paar et al., 2006].All are tied to the Bi-mineral associations. One of the striking aspects is thewidespread occurrence of native gold within symplectites together withnative bismuth, wheremaldonitemay be present either in adjacent areasorwithin the symplectites themselves (Fig. 4a, b).Wenote the differencesin themorphology of the boundaries of the symplectites, i.e. interlaced orsharp towards themaldonite or pre-existing native bismuth, respectively.The Ag content of gold is commonly low (mean 0.41wt.%), but somelarger, marginal areas of the symplectites contain Ag up to 7.37wt.%(Table 2).

In contrast to the aspects described above, equilibrium relationshipsbetween maldonite and native bismuth and phases from the tetra-dymite groupare seenwithin the smaller, composite Au–Bi–(Te,S) blebs(Fig. 4c–f). In a few of them, the association of maldonite, native goldand bismuth has also been observed (Fig. 4e). A variety of aurostibitewith high Bi content (mean 9.42wt.% Bi; Table 2) has also beenidentified from several blebs in the same sample (Fig. 4f).

Jonassonite has been identified as small, stubby to irregular grains(up to 50 μm; Fig. 5a–e). Most of these are hosted within one ofthe largest Au–Bi–Te–S patches, marginally corroded by chlorite–bismuthinite inliers (Fig. 3a). At Maldon, jonassonite contains eitherlow Pb or none (Table 2); we thus use the simplified formula, AuBi5S4,for the mineral in this paper. Compositional variation in jonassonitefrom different deposits worldwide has been discussed by Ciobanuet al. (2006b). The jonassonite grains occur along the boundarybetween native bismuth in the inner part of the patch and an envelopeof sulphotellurides. The irregular contact between this envelope andnative bismuth, as well as the presence of patchy areas of nativebismuth and of occasional, thin rods of native gold throughout theenvelope (Fig. 5a), indicate that the sulphotelluride+jonassoniteassociation results from the replacement of native bismuth associatedwithmaldonite. Thus fluids associatedwith stage 2 (sulphotelluride+

Bi-tellurides (tetradymite group) BDs

Asb Hed Unnamed Unnamed JoB JoA Bism

AuSb2 Bi7Te3 Bi6Te2S Bi3(Te,S)2 Bi4Te2S Bi4TeS2 Bi2S3

x x xx x x x x x

X XX x x xx X X x

e CSIRO collections housed at the Geology Department, University of Melbourne.

, JoA—joséite-A, BDs—bismuthinite derivatives, Bism—bismuthinite, x scarce, X widespread.

Table 2Electron probe microanalytical data for gold minerals (selected points and means).

Native gold(m1)

Mean (n=7) Native gold(m2)

Mean (n=4) Ag-rich (m1) Maldonite (m1) Mean (n=10) Maldonite(m2)

Mean (n=5)

Analysis (wt.%)Au 99.62 98.88 100.09 101.07 98.52 100.46 92.80 65.38 65.99 65.86 65.03 63.93 64.56Ag 0.81 0.74 0.43 – 1.07 0.50 7.37 0.03 – 0.31 0.01 – 0.05Pb – – – – – 0.12 – – – 0.07 – – –

Bi 0.36 0.43 0.51 0.47 0.60 0.79 0.41 36.01 35.23 35.78 35.33 35.32 35.74Sb – 0.04 0.03 – – 0.02 0.05 – 0.05 0.05 – – –

As – 0.02 0.02 0.06 – – –

Hg 0.27 – 0.27 – – 0.01Te – 0.07 0.04 0.03 – 0.07 – – – 0.04 0.11 – 0.11Se – – 0.02 0.01 – 0.02 0.03 – – 0.02 0.04 – 0.02S – 0.02 0.03 0.01 0.05 0.02 0.02 – – – 0.04 – 0.04Total 100.78 100.21 101.10 101.86 100.24 101.79 100.74 101.42 101.26 102.04 100.56 99.25 100.38

Formulae (On basis of 1 a.p.f.u.) (On basis of 3 a.p.f.u.)Au 0.982 0.979 0.986 0.992 0.972 0.983 0.867 1.973 1.994 1.970 1.974 1.973 1.967Ag 0.015 0.013 0.008 – 0.019 0.009 0.126 0.002 0.017 0.001 0.002Au+Ag 0.997 0.993 0.993 0.992 0.991 0.989 0.992 1.975 1.994 1.987 1.974 1.973 1.969Pb – – – – – 0.001 – – – 0.001 – – –

Hg 0.003 – 0.003 – – 0.000Sb – 0.001 0.000 – – 0.000 0.001 – 0.002 0.002 – – –

As – 0.001 0.001 0.001 – – –

Bi 0.003 0.004 0.005 0.004 0.006 0.007 0.004 1.025 1.003 1.009 1.011 1.027 1.026∑Me 1.000 0.998 0.999 0.999 0.997 0.998 0.998 3.000 3.000 2.998 2.985 3.000 2.995Te 0.000 0.001 0.000 0.000 0.000 0.001 0.000 – – 0.001 0.005 – 0.001Se – – 0.000 0.000 – 0.001 0.001 – – 0.001 0.003 – 0.001S – 0.001 0.002 0.000 0.003 0.001 0.001 – – – 0.007 – 0.003∑(S,Se,Te) 0.000 0.002 0.001 0.001 0.003 0.002 0.002 0.000 0.000 0.002 0.015 0.000 0.005

Aurostibite (m2) Jonassonite (m3) Mean (n=4) Jonassonite (m5) Mean (n=16)

Analysis (wt.%)Au 43.03 43.32 44.05 14.05 14.21 14.19 15.33 15.32 15.46 15.00 15.30Ag – – 0.05 0.08 0.01 0.05 – – – – 0.07Pb – – – 2.37 1.78 1.42 – – – – 0.00Bi 9.06 9.65 9.55 73.67 74.66 75.78 75.35 75.61 74.83 75.84 76.44Sb 47.38 47.89 47.56 0.36 0.28 0.36 – – 0.02 – 0.06Hg – – 0.12 – – 0.10 – 0.10 – – 0.17Te – – – – 0.07 0.07 – 0.08 0.03 0.02 0.04Se – 0.08 0.05 0.02 0.03 0.03 0.01 0.06 – 0.04 0.03S – 0.03 – 9.49 9.70 9.45 9.49 9.59 9.68 9.67 9.65Total 99.48 100.96 101.38 100.04 100.76 100.93 100.18 100.76 100.01 100.56 101.52

Formulae (On basis of 3 a.p.f.u.) (On basis of 10 a.p.f.u.)Au 1.007 0.998 1.014 0.970 0.970 0.976 1.060 1.050 1.062 1.028 1.042Ag – – 0.002 0.011 0.001 0.006 – – – – 0.009Au+Ag 1.007 0.998 1.016 0.981 0.971 0.979 1.060 1.050 1.062 1.028 1.043Pb – – – 0.155 0.115 0.093 – – – – –

Hg – – 0.003 – – 0.007 – 0.007 – – 0.005Sb 1.793 1.785 1.771 0.040 0.031 0.041 0.000 0.000 0.002 – 0.003Bi 0.200 0.210 0.207 4.795 4.802 4.913 4.910 4.886 4.847 4.897 4.907∑Me 3.000 2.992 2.997 5.971 5.919 6.004 5.970 5.943 5.911 5.924 5.958Te – – – – 0.007 0.007 0.000 0.009 0.003 0.002 0.001Se – 0.004 0.003 0.004 0.006 0.005 0.001 0.009 – 0.006 0.004S – 0.004 – 4.025 4.068 3.992 4.030 4.039 4.086 4.068 4.037∑(S,Se,Te) 0.000 0.008 0.003 4.029 4.081 3.996 4.030 4.057 4.089 4.076 4.042

–: Concentration below detection limit (analysis wt.%) or insignificant at 3 decimal places (formulae), blank: not analysed.


jonassonite) have introduced Te and some S, and have reacted withthe stage 1 association (bismuth+maldonite); see discussion. Thisinterpretation is further stressed by the fact that the sulphotelluridesand jonassonite extend pervasively along cracks and/grain boundariestowards the inner part of the native bismuth (Fig. 5b).

Although replacement by bismuthinite affects the entire marginalarea of the patch, there is a more pronounced reaction with the Au-bearing minerals. Inclusions of bismuthinite within jonassonite andvice versa (Fig. 5c, d) can be considered as representing initial andfinal stages of replacement of jonassonite by bismuthinite, respec-tively. Both minerals can, however, co-exist as minute inclusionsalong cracks/cleavage planes within native bismuth (Fig. 5d), imply-ing a locally-buffered environment.

Total replacement of jonassonite may also result in dense, minuteinclusions of native gold spread throughout the bismuthinite (Fig. 5e).The reverse reaction, i.e. jonassonite forming from bismuthinite andgold, has also been observed in patches within chlorite.

3.4. Phases from the tetradymite group

Five phases from the tetradymite group: joséite-B, joséite-A, twounnamed phases with compositions ∼Bi3(Te,S)2 and ∼Bi6Te2S (butsee below) and hedleyite are identified in the analysed material(Table 4). Each of these phases, except for joséite-A, forms compositeblebs with native bismuth and/or maldonite, and rarely also gold(Fig. 4d, e). Hedleyite, together with bismuth, has only been observed

Table 3Electron probe microanalytical data for native bismuth and bismuthinite (means and single points).

Native bismuth Bismuthinite Sb-bearing bismuthinite

m1Mean (n=6)

m2Mean (n=7)

m3(n=1)

m1Mean (n=4)

m2Mean (n=2)

m3(n=1)

m5Mean (n=2)

m1Mean (n=3)

m5Mean (n=6)

Analysis (wt.%)Au 0.02 0.03 1.61Ag 0.02 0.01 – – – – 0.01 – 0.02Cu – – – – – –

Pb – – 0.06 0.03 – – – 0.05 –

Bi 100.24 98.83 95.95 81.61 81.09 80.73 80.39 73.41 77.27Sb 0.42 0.99 0.07 0.24 0.46 1.22 1.37 7.31 4.31Hg – – 0.07Te 0.01 0.01 – 0.07 0.04 0.01 0.01 – 0.02Se 0.02 0.02 – 0.01 0.03 – – 0.04 0.02S 0.01 – 0.89 18.61 19.08 17.24 19.31 19.81 19.68Total 100.72 99.90 98.64 100.57 100.70 99.35 101.09 100.62 101.32

Formulae (On basis of 1 a.p.f.u.) (On basis of 5 a.p.f.u.)Au – 0.001 0.016Ag – – – – – – – – 0.001Cu – – – – – –

Pb – – 0.001 0.001 – – – 0.001 –

Hg – – 0.001Bi 0.991 0.981 0.925 2.006 1.964 2.062 1.927 1.706 1.814Sb 0.007 0.017 0.001 0.010 0.019 0.053 0.056 0.291 0.173∑M 2.017 1.984 2.128 1.983 1.998 1.988Te – – – 0.003 0.002 – – – 0.001Se 0.001 0.001 – 0.001 0.002 – – 0.002 0.001S – – 0.056 2.979 3.013 2.871 3.016 2.999 3.010∑(S+Se+Te) 2.983 3.016 2.872 3.017 3.002 3.012

–: Concentration below detection limit (analysis wt.%) or insignificant at 3 decimal places (formulae), blank: not analysed.There is some rounding of the totals to 2 and 3 decimal places for the wt.% analysis and formulae, respectively.

Table 4Electron probe microanalytical data for phases from the tetradymite group (selected points (p) and means (m)).

Hedleyite Bi6Te2S ∼Bi3TeS Joséite-B Joséite-A

m2.1(n=2)

m1.1(n=2)

p2.2(n=1)

p2.3(n=1)

m4.1(n=8)

p5.1(n=1)

m1.2(n=10)

p1.3(n=1)

p2.4(n=1)

m3.1(n=12)

p4.2(n=1)

m5.2(n=27)

m4.3(n=5)

m5.3(n=14)

Analysis (wt.%)Ag – – 0.08 – 0.02 – 0.03 0.07 – 0.02 – – – –

Pb – 0.13 – – – – – 0.13 – 0.07 – 0.03 – –

Bi 78.16 79.90 76.47 77.88 80.40 81.13 75.45 75.35 76.26 75.70 76.69 76.38 80.97 82.50Sb 1.19 1.15 0.84 0.74 0.63 0.63 0.52 0.52 0.48 0.32 0.34 0.32 0.44 0.42Te 19.78 16.20 17.56 17.61 15.75 15.31 21.25 21.34 20.79 21.16 22.01 21.49 12.33 11.83Se 0.12 0.53 0.19 0.25 0.32 0.27 0.52 0.61 0.24 0.14 0.14 0.49 0.24 0.20S 0.00 2.02 2.21 3.42 3.75 3.85 2.88 2.80 2.92 2.97 2.94 2.92 6.11 6.56Total 99.25 99.93 97.35 99.89 100.86 101.20 100.64 100.81 100.69 100.38 102.12 101.65 100.09 101.52

Formula (On basis of 10 a.p.f.u.) (On basis of 9 a.p.f.u.) (On basis of 5 a.p.f.u.) (On basis of 7 a.p.f.u.)Ag – – 0.01 – 0.001 – 0.003 0.01 – 0.002 – – – –

Pb – 0.01 – – – – – 0.01 – 0.004 – 0.002 – –

Bi 6.92 5.86 5.65 2.97 3.03 3.05 4.02 4.02 4.08 4.05 4.04 4.03 3.98 3.96Sb 0.18 0.15 0.11 0.05 0.04 0.04 0.05 0.05 0.04 0.03 0.03 0.03 0.04 0.03∑M 7.10 6.02 5.77 3.02 3.07 3.09 4.07 4.08 4.13 4.09 4.07 4.07 4.02 3.99Te 2.87 1.94 2.13 1.10 0.97 0.94 1.86 1.86 1.82 1.85 1.90 1.86 0.99 0.93Se 0.03 0.10 0.04 0.02 0.03 0.03 0.07 0.09 0.03 0.02 0.02 0.07 0.03 0.03S 0.00 0.94 1.06 0.85 0.92 0.94 1.00 0.97 1.02 1.04 1.01 1.01 1.95 2.05∑X 2.90 2.98 3.23 1.98 1.93 1.91 2.93 2.92 2.87 2.91 2.93 2.93 2.98 3.01

Formula recalculated to X=3Ag – – 0.01 – 0.002 – 0.003 0.01 – 0.002 – – – –

Pb – 0.01 – – – – – 0.01 – 0.004 – 0.002 – –

Bi 7.17 5.84 5.26 4.51 4.72 4.78 4.12 4.13 4.26 4.17 4.14 4.13 4.00 3.95Sb 0.19 0.14 0.10 0.07 0.06 0.06 0.05 0.05 0.05 0.03 0.03 0.03 0.04 0.03∑M 7.36 5.99 5.37 4.59 4.79 4.85 4.17 4.19 4.31 4.21 4.17 4.16 4.04 3.98Te 2.97 1.94 1.98 1.67 1.51 1.48 1.90 1.91 1.90 1.91 1.95 1.90 1.00 0.93Se 0.03 0.10 0.03 0.04 0.05 0.04 0.08 0.09 0.04 0.02 0.02 0.07 0.03 0.03S 0.00 0.96 0.99 1.29 1.44 1.48 1.02 1.00 1.06 1.07 1.03 1.03 1.97 2.05∑X 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00

X=Te+S+Se.–: Concentration below detection limit (analysis wt.%) or insignificant at 3 decimal places (formulae), blank: not analysed.There is some rounding of the totals to 2 and 3 decimal places for the wt.% analysis and formulae, respectively.


Fig. 3. Back-scattered-electron (BSE) images showing the occurrence of patches and blebs of Au–Bi–Te–Smineral assemblages, their textural relationships and overprint features. (a)Representative large patch with zonation pattern: inner bismuth (Bi) relict enveloped by sulphotellurides+jonassonite (Jon), with an outer zone of Au–Bi symplectite containingrelict maldonite. The outer zone shows pronounced replacement by bismuthinite+chlorite. Detail in rectangle is shown in Fig. 3e. Circles represent areas of larger jonassonite grains,including those shown in other figures as marked. (b) Field of blebs (some shown in Fig. 4 as marked) within quartz. Note cross-cutting veinlets and pressure gashes along them;rectangle indicates detailed image of these in Fig. 3c. Asb—aurostibite, Au—native gold, Bi—native bismuth, Hed—hedleyite, Mld—maldonite, tell—Bi-(sulpho)tellurides. (c) Typicalassemblage in pressure gashes consisting of chlorite (Chl) intergrown with bismuth+bismuthinite (Bism). (d) Patchy relict Au–Bi–(Te–S) minerals embedded within altered K-feldspar (Kfs)+biotite (?) hornfels; alteration by chlorite. Note large grains of apatite (Ap). Inset shows typical relict maldonite enveloped by gold and bismuthinite. (e) Detailshowing replacement of Au–Bi–Te–S assemblage by chlorite+bismuthinite. The contact between the alteration inlier and corroded patch is marked by bismuthinite. JoB—joséite-B.


in blebs. Equilibrium crystallisation between the phases is underlinedbymutual boundaries featuring triple junctions in themulti-componentblebs (e.g. Fig. 4d).

In the larger patches, the sulphotellurides are often associated withone another as the main constituents, alongside native bismuth (Figs. 3a

and 5f). In the present samples, either joséite-B or the ∼Bi3(Te,S)2 phaseis dominant. In the latter case, both joséite-B and -A surround clots ofbismuth within the host ∼Bi3(Te,S)2. This may be indicative of replace-ment of one sulphotelluride species by another, either by solid statediffusion (most likely) or by fluid-driven infiltration.

Fig. 4. (a) BSE image showing symplectites of gold and bismuth (black arrows) at the contact betweenmaldonite (Mld) and bismuth (Bi). A lamella of joséite-B (JoB) is visible at one of themargins. Note replacement inliers by chlorite (Chl) and bismuthinite with relicts of Au–Bi–Te–S minerals (bright). (b) Detail of rectangle shown on Fig. 4a showing the sharp contactbetween symplectite and bismuthwhich contrasts with the interlaced contact towards themaldonite. Relict areas ofmaldonite are also presentwithin the symplectites. (c–f) BSE imagesshowing blebs where bismuth is associated with maldonite (in c), maldonite and Bi3(Te,S)2 (in d), Au+joséite-B+maldonite (in e) and with aurostibite (in f). The assemblage in (e),although composed of the same minerals as in (a), the grain contact between the sulphotellurides and Au–Bi minerals is not sharp, suggesting crystallisation frommelt.


Compositional data for sulphotellurides and hedleyite are sum-marised in Table 4 and Figs. 6 and 7. In the table, the data are presentedfollowing current practice, with formulae calculated to reflect thesmallest integer number of atoms in the formula unit (Cook et al.,2007a). Accordingly, and despite some variation in the ratio betweenmetals and chalcogens, unnamedBi3TeS andBi6Te2S canbe calculated to5 and 9 a.p.f.u. (atoms per formula unit), respectively. To give moreinsight about the identity of these phases and for comparison amongthem, we have recalculated all analysed points to the number of metals(M) relative to (Te,Se,S)=3. According to Ciobanu et al. (2009b), thisallows for calculation of the stacking sequence.

For all points in our dataset, M is between 4 and 7 with significantvariation for each mineral or phase (Fig. 7a). For joséite-B, M varieswithin the range of 3.8 to 4.5 in each of the analysed samples. In thecase of joséite-A, the stoichiometry is generally close to, or slightlylower (M=3.8) than the ideal (M=4) in the analysed samples, withsome few outliers (M∼4.5). For unnamed Bi3(Te,S)2 there is a

discrepancy between the blebs and the patches. In the blebs, M variesfrom the ideal 4.5 up to 4.8, whereas in the patches, M is always closeto 4.8, corresponding more closely to Bi8(Te,S)5. Our few analyses ofthe unnamed ∼Bi6Te2S phase show M close to the ideal 6, except forone point (M=5.3). M values for hedleyite are between 7.25 and 7.5(i.e. higher than the ideal value of 7). Based on previously publisheddata for the tetradymite group (Cooket al., 2007a, and references therein),these ranges of deviation from ideal stoichiometry are not unusual.Sometimes, they are associated with stacking disorder and the latter canprovide insights into equilibrium conditions (Ciobanu et al., 2009b).

Except hedleyite, all other phases from the tetradymite group presentat Maldon contain variable amounts of S; Se is negligible in all cases(Table 4). As shown for M above, there is also systematic variation in Tecontent relative to total chalcogens (Fig. 7b) for all phases except Bi6Te2Sand hedleyite. Ranges for joséite-A and -B are comparable from onesample to another. Both minerals are slightly deficient in Te relative tothe ideal formulae (Bi4TeS2 and Bi4Te2S, respectively), with means that

Fig. 5. (a–e) BSE images showing aspects of the Bi-sulphotelluride envelope to native bismuth in the patch shown in Fig. 3a where jonassonite is also present. (a) Stubby grain ofjonassonite (Jon) positioned between joséite B (JoB) and bismuth (Bi). Note the thin rod of native gold along a fracture with bismuthinite. (b) Pervasive replacement of the nativebismuth by joséite-B and jonassonite in the inner part of the patch. (c) Inclusions of bismuthinite within jonassonite and vice versa (in d), interpreted as initial and final stages ofreplacement of jonassonite by bismuthinite, respectively. In (d), inclusions of jonassonite and bismuthinite (note tendency towards euhedral grain morphology of both) in nativebismuth points to the fact the latter locally buffers their co-existence. Gn: galena. (e) Pervasively distributed grains of native goldwithin bismuthinite resulting from the replacementof jonassonite. Note also Sb-rich zones within the bismuthinite that does not host native gold. (f) Large patch of Bi-sulphotelluride with composition close to Bi3(Te,S)2 (meancomposition Bi8(Te,S)5 see text for explanation) and native bismuth. Note clots of joséite-A and -B and native bismuth embedded within the host sulphotelluride.


nevertheless plot close to Te=1 and 2 a.p.f.u. (Fig. 7b). Bi3TeS showsperfect Te:S stoichiometry (Te=1.5) in the patches, but TeNS in the blebs(Fig. 7b).

In terms of other elements in theseminerals, we note the scarcity ofPb in joséite-A and -B, which generally carry up to several wt.% ofthis element (Cook et al., 2007a). Antimony is typically present in allphases (Table 4), with concentrations highest in hedleyite andincreasing withM (Fig. 7c). Bi3TeS from patches and blebs both containsimilar ranges of Sb variation, contrasting with the distinctive M andTe variation trends.

4. Transmission electron microscopy

The unnamed Bi3(Te,S)2 phase shows marked chemical differencesbetween the patches and the blebs. Together with the textural evidence

for overprinting in the patches, identification of the stacking sequencescould provide insight into the identity of the phase and the conditions offormation in equilibriumwith joséite-A, -B and bismuth. The coarse patchin Fig. 5f was chosen for TEM investigation. Themethods of Ciobanu et al.(2009b) were used to characterise this phase. The approach involves twosteps: (i) calculation of the stacking sequences from the compositions and(ii) comparison with those estimated from electron diffraction patterns.

4.1. Calculating stacking sequences from compositions

Using the structural formula: S′(Bi2kX3)·L′(Bi2(k+1)X3), where S′ andL′ are the number of short and long modules, respectively, the end-rangephases (joséite and hedleyite) are characterised by k=2, S′=1, L′=0(sequence 7′) and k=3, S′=L′=1 (sequence 9′11′), respectively. Fromthe compositional data for the Bi3(Te,S)2 phase in the chosen patch (in

Fig. 6. Ternary diagram (Bi+Sb–S+Se–Te) showing the compositions of mean (m)and selected points (p) from Bi-sulphotellurides and hedleyite in each analysed sample(see Table 4). Dashed lines show known and inferred isoseries in the tetradymite group(Bi4X3, Bi4.5X3, Bi5X3, Bi6X3 and Bi7X3, where X is the chalcogen). Dotted lines representthree (Te:S+Se) ratios (2:1, 1.5:1.5 and 1:2). Circles represent data from patches,diamonds show data from blebs. Grey stars indicate mineral phases (Ik—ikunolite, JoAand JoB—joséite-A and -B, Pls—pilsenite, Hed—hedleyite). White stars indicate idealBi3TeS and Bi6Te2S. Grey arrow represents the progressive enrichment of sulphur in thephases in the assemblages.

Fig. 7. Compositional plots showing variation among individual data points alignedalong the horizontal axes for each species in each sample. Values represented on thevertical axis [M in (a), Te in (b) and Sb in (c)] are calculated in terms of X=3 a.p.f.u.(atoms per formula unit) for each species, where M is Bi(+Sb+Pb+Ag) and X=Te+S+(Se). Circles represent data from patches, diamonds show data from blebs. Solidvertical lines separate distinct species, dotted vertical lines separate samples. In (a), thedotted horizontal lines represent ideal stoichiometries for the isoseries shown in Fig. 6.Data in the rectangle in (a) refer to the main sulphotelluride in the patch on Fig. 5f withmean M=4.8. In (b), dotted horizontal lines represent ideal (Te:S+Se) ratios as inFig. 6. In (c), dotted lines are ranges of Sb content for each species showing theincremental increase in Sb content with M relative to X=3.


which M is 4.8 rather than the ideal 4.5, i.e. Bi8X5), the stacking sequencewould correspond to k=2, S′=3 and L′=2. The stacking sequences forthe ideal phases relevant here are shown on Fig. 6.

Oneof thepossible stacking sequences forBi8X5 combines7′7′9′and7′9′ sub-sequences, corresponding to Bi14X9 and Bi5X3, respectively, withMvalues for the two sub-sequences corresponding to 4.67 and 5.0 (Table 5).Even though each of these two sub-sequences is simpler than the onecorresponding to Bi3X2 (7′7′7′9′), the lengths of the c parameter arelonger than forBi3X2 (138 and96 Åcompared to60 Å)due to thedifferingrhombohedral and trigonal symmetries, respectively.

4.2. Electron diffraction patterns

The electron diffractions patterns were calculated for Bi8X5 and thephases Bi14X9 and Bi5X3 corresponding to the sub-sequences discussedabove using MSCD software (Appendix 3 in Ciobanu et al., 2009b).The relevant intensity values are listed in Electronic Appendix A.

The FIB technique was employed to cut a slice, 25 µm in length,12 µm in width and 5 µm depth, through the patch, half way betweenthe contactwith native Bi and the edge of the sample. The location of theFIB cut and several steps of the procedure are shown in Fig. 8. Thethickness of foil could not be brought below the ∼100 nmnecessary forHR-TEM imaging, but nonethelesswas adequate for electron diffraction.

Two types of characteristic electron diffraction pattern (EDP) wereobtained from the [−110]R=[110]h zone axis (selected areas ofelectron diffraction, SAEDs in Fig. 9a, b). Three-integer indexation,corresponding to the subscript R, is given with respect to arhombohedral subcell and (3+1) integer (hklm) indexation is givenwith respect to the hexagonal cell setting (subscript h). The EDPsweretaken at locations along the edge of the FIB-TEM slice (Fig. 8d). Theyshow the co-existence of two distinct, ordered phases with parallelorientation to one another. The total number of atoms and modulesin the unit cell is represented by the number of divisions in the d*interval and between the two reflections in the middle of this interval(Fig. 10a, b and respective models in Fig. 10c, d). Counting

these divisions on EDPs we conclude that the two phases are 16-and 23-fold superstructures built by 2- and 3-modules, respectively.Considering also the lengths of these divisions within the d* interval,the stacking sequences for the two phases are 7′9′ and 7′7′9′,respectively. The results are also confirmed by the values obtained for

Table 5Stacking sequence calculations from chemical data for the unnamed phase studied byTEM.

FormulaeMean m4.1 (n=9) recalculated to

Bi3X2

5 a.p.f.u.Bi14X9

23 a.p.f.u.Bi8X5

13 a.p.f.u.Bi5X3

8 a.p.f.u.

Bi 3.03 13.92 7.87 4.84Sb 0.04 0.19 0.10 0.06Total M 3.07 14.11 7.98 4.91Te 0.95 4.38 2.47 1.52Se 0.03 0.14 0.08 0.05S 0.95 4.37 2.47 1.52Total X (Te+S+Se) 1.93 8.89 5.02 3.09Calculated M when X=3 4.5 4.67 4.8 5Recalculated for N in stacking sequence Bi18X12 Bi14X9 Bi24X15 Bi10X6

N 30 23 39 16k 2 2 2 2S′ 3 2 3 1L′ 1 1 2 1S′+L′ 4 3 5 2

A AB BStacking sequence 7′7′7′9′ 7′7′9′ 7′7′9′.7′9′ 7′9′c parameter length (Å) 60 138 78 96

N=total number of layers in stacking sequence.k=structural factor (see text).S′ and L′=number of short and long modules.7′=short module of the type: X–Bi–X–Bi–X–Bi–Bi; 9′=long module of the type: X–Bi–X–Bi–X–Bi–Bi–Bi–Bi.

Fig. 8. Secondary electron images showing several stages during preparation of a TEM foil usilocation of the slice from patch in Fig. 5f. The morphology of the patch is modified after extracprepared by microtome thinning; the holes are laser-ablation ICP-MS pits (see Ciobanu et al., 2


the modulation vector q along the c* axis for the two phases (Fig. 9);see Lind and Lidin (2003) and Ciobanu et al. (2009b).

The above results indicate that the phase is not homogenous, butinstead features coherent lattice-scale intergrowths of the two phases(ideally Bi14X9 and Bi5X3). Although each of the sequences is regular,without observable disorder on the EDPs, their distribution is irre-gular across the foil, leading to the conclusion that the phase withmean composition Bi8X5 represents a disordered intergrowth ofsmall slabs of two ordered polysomes (homologues). In contrast, aregular intergrowth of the two stacking sequences identified(corresponding to the same Bi8X5 composition) would have a distinctdiffraction pattern (asmodelled in Fig. 10e) but this was not observed.This patterns would have 39 divisions within the d* interval with 5divisions between the two bright reflections in the middle part. Themarked differences between the stacking sequences representingBi14X9, Bi5X3 and Bi8X5 are seen in the models (Fig. 10c–e) based onMSCD calculations (Appendix A). This sequence has a shorter c length(78 Å) than either Bi14X9 or Bi5X3. Such irregular intergrowths betweentwo types of ordered stacking sequences represent a case of polysomatic(homologue) disorder analogous to that found elsewhere for anotherphase with composition close to Bi3Te2, in which the same sub-sequences could be identified (Ciobanu et al., 2009b).

The results indicate the presence of nanoscale domains of Bi14X9

and Bi5X3, each at least several hundred nm in length if we considerthe necessity of having domains of the order of tens of unit cells toobtain EDPs showing ordered stacking sequences. Such domains arecompatible with local equilibrium conditions since the modules that

ng FIB technique, as written on each. The rectangle in (a) and blown-up in (b) show thetion of material for an unsuccessful attempt at identifying the sulphotelluride in samples009a). Circles in (d) are the selected areas of electron diffraction (SAED) shown in Fig. 9.


build both stacking sequences have structural factors k withconsecutive numbers, i.e. 2 and 3 for modules 7′ and 9′, respectively.The same composition (Bi8X5) recalculated to correspond to N=39reflecting the stacking sequence (i.e. Bi24X15) could be expressed as anintergrowth of other sub-sequences, e.g. (2×5)(2×9′11′)(1×11′)corresponding to a combination of Bi2X3, Bi6X3 and Bi8X3. Such anintergrowth could not form at equilibrium because themodules in thestacking package will not have consecutive k values (7′ is missing).The intergrowths we have identified may represent an intermediatestage of replacement, developed at local equilibrium conditions,between a phase with higher M value (hedleyite or Bi8Te3?) andspecies from the isoseries 4:3 (structure 7′) such as joséite-B and -A.

5. Modelling formation of the Au–Bi–Te–S assemblages

5.1. Melt hypothesis

Thebismuth-richblebs atMaldon canbeat least partially explainedbycrystallisation of melts from the system Au–Bi–Te. If so, the associations

Fig. 9. Selected areas of electron diffraction (SAED) along the [−110]R=[110]h zone axis

representative for Bi5(Te,S)3 (in a) and Bi14(Te,S)9 (in b). Three-integer indexation,corresponding to the subscript R, is given with respect to a rhombohedral subcell and(3+1) integer (hklm) indexation is given with respect to the hexagonal cell setting(subscript h). Single-tipped arrows represent the q modulation vector along c* withvalues as indicated on the figures. The interval d* (1/d*∼0.2 nm) is characteristic for allphases in the tetradymite group. Distribution of reflections within this interval(rectangular boxes) along the central row differs from (a) to (b) and has been used toidentify the two stacking sequences (see Fig. 10 and text for more explanation). Thetwo types of diffraction (a for SAED1 and b for SAED 2) alternate along the foil in Fig. 8d.

will include phases representing eutectic assemblages formed at theend of partial crystallisation along the solvus curve (Ciobanu et al., 2005).On the Bi-rich side of the system Au–Bi–Te there are three eutectics:bismuth+Bi7Te3 (hedleyite) (266 °C), Au2Bi (maldonite)+bismuth(241 °C) and Au2Bi+bismuth+Bi7Te3 (235 °C) (e.g. Prince et al.,1990). Associations representing the first two eutectics are clearly ob-served in the blebs at Maldon (e.g. Fig. 4c for the second of these). Thereare no published data for the 4-component system Au–Bi–Te–S and theBi:chalcogen stoichiometry of the telluride in the ternary eutectic is also amatter of discussion. This was given as either Bi5Te3 or Bi7Te3, based onexperimental work or thermodynamic re-evaluation of the Au–Bi–Tesystem, respectively (see discussion in Prince et al., 1990); other, Bi-richerphases recently identified (e.g. Bi8Te3; Ciobanu et al., 2009a) could also beconsidered for the same eutectic. Therefore the variable Bi:chalcogenstoichiometry in the observed tellurides in the blebs at Maldon, and thefact that the majority of them are S-bearing species (e.g. Fig. 4d), are not

Fig. 10. (a) and (b) Strips from the central rows of four EDPs, showing distribution ofreflections within d* intervals representative for the phases shown in Fig. 9a and b,respectively. Two examples are shown in each case in order to illustrate a morecomplete series of reflections. Note the presence of the most intense superstructurereflections as a pair in the middle of d* and the slight increase in the distance betweenthem from (a) to (b), corresponding to a decrease in the Bi:chalcogen ratio. Arrowspoint to the smallest interval identified in the strips, decreasing in size from (a) to (b).This, together with the higher number of reflections in (b), indicates higher N for (b)compared to (a); see models in (c) and (d), respectively. (c–e) Modelled distribution ofreflections in d* intervals using intensity values computed by MSCD (Appendix A) forphases as written on the individual sub-figures. Note the similarity between the modelin (c) and (d) and the strips in (a) and (b), respectively. The model in (e) represents aphase with an average composition between those in (c) and (d) and differs in thenumber of intervals along d* (39) relative to 16 and 23 in (c) and (d), respectively, andthe number of divisions between the two main reflections in the middle of d*, i.e. 5relative to 2 in (c) and 3 in (d).


necessarily arguments against their formation frommelts and that thosemelts must have contained S.

Even though most of the blebs appear Bi-rich and containmaldonite without native gold, suggesting that crystallisation startedwith bismuth, some do include gold (Fig. 4e). Considering a meltscenario, these must have been formed from Au-rich melts. In such acase, the sequence of partial crystallisation, supported by preservedtextures, would have started with gold above the peritectic at 371 °C(Okamoto and Massalski, 1983), and then followed the solvuscrystallising maldonite that re-equilibrates partially with gold, andfinally, bismuth in equilibrium with maldonite (241 °C). In suchblebs, a phase close to joséite-B has also been observed, indicating amore complex melt.

Although dominated by a bismuth–maldonite association, thepresence of bismuth with aurostibite further indicates mm-scalecompositional variation in the area containing the blebs, andimplicitly that the melts had a significant Sb content. Experimentsin the system Au–Bi–Sb (see Prince et al., 1990) show that AuSb2 inequilibrium with Au–Bi–Sb melt can incorporate as much as 4.5wt.%Bi at 238.6 °C. The higher amount of Bi in aurostibite from Maldon(∼9wt.%) could indicate a temperature higher than this, assumingthat the Sb content has been accurately measured despite the smallgrain size.

A strong argument favouring the melt scenario is that maldonite,aurostibite or any of the chalcogenides of bismuth are never seenalone. They are always components of the bismuth blebs. Based onthe above, the lower limit of the temperature interval for meltformation can be constrained at 235 to 371 °C, well below the lowerlimit of peak conditions in the contact metamorphic aureole(≥500 °C; Hack et al., 1998).

5.2. Overprinting

In contrast to the smaller blebs, Au–Bi–Te–S assemblages in thepatches from Maldon show clear evidence of textural overprinting,such as replacement by bismuthinite (+ chlorite), as well as indirectevidence for local re-equilibration of phases. Despite the complexityof these assemblages as seen now, the sequence of overprintingsuggests that they may have originally been analogous in composi-tion to the type of relatively simple binary or ternary eutecticsdescribed for the blebs above. Their reworking could have takenplace either during cooling, interaction with fluids and/or by furtherpartial melting. The latter two can easily be linked to the protractedgeological history of Maldon (see below). The importance of reactionwith S-bearing fluids is underlined by the presence of several S-bearing telluride species, together with the S-bearing Au-phasejonassonite. A dominant feature of the overprint in the patches is thereplacement of precursor maldonite by symplectites of gold andbismuth. Further results of the overprinting include variation inmineral chemistry (e.g. Ag in gold or Sb in bismuthinite).

An example of local re-equilibration of phases during interac-tion with fluids is provided by phases from the tetradymite group,which typically occur as combinations of several distinct specieswithin a given patch, thus contrasting with the single species seen inthe blebs. Illustrative of this is the association between bismuth andboth joséite-A and -B, together with the polysome representing theBi8(Te,S)5 phase (Fig. 5f). Sulphidation of an initial bismuth+hedleyite assemblage formed at the binary eutectic could result ina 3-component sulphotelluride assemblage following reaction oftype (1a) or (1b):

5Biþ 3Bi7Te3 þ 9SO2�4 þ 18H

þ ¼ 2Bi5Te1:5S1:5 þ 2Bi4Te2Sðjose0 ite�BÞþ2Bi4TeS2ðjose0 ite�AÞ þ 9H2Oþ 13:5 O2

ð1aÞ

5Biþ 3Bi7Te3 þ 9H2S ¼ 2Bi5Te1:5S1:5 þ 2Bi4Te2Sðjose0 ite�BÞþ 2Bi4TeS2 ðjose0 ite�AÞ þ 9H2 ð1bÞ

The two reactions refer to the oxidation state of sulphur in the fluid(left side of the equations), i.e. high- (S6+, 1a) and low-sulphidation (S2−,
1b) of the sulphur. The same also applies to reactions (2a,2b)–(5a,5b).Such a reaction explains the derivation of diverse S-bearing species froma Bi-rich, Te-poor precursor (arrow on Fig. 6).
Replacement of a hypothetical hedleyite precursor formed at theeutecticwith bismuth can be locally equilibrated by formation of lattice-scale intergrowths, given the polysomatism in the tetradymite group.Stacking sequences indeed indicate the presence of Bi5(Te,S)3 in one ofthe sub-sequences documented and interpreted from the EDPs (Figs. 9and 10). Local equilibrium in the assemblage is indicated by the fact thatthe structural modules in the polysome have k values decreasingstepwise towards chalcogen-rich compositions (from 3 to 2 betweenBi7Te3 and Bi4Te3), thus gradually adjusting the compositional changefrom bismuth+hedleyite towards Bi4Te3 as in reactions (1a) and (1b).This is reflected in the fact that a common type of module (9′) ispreserved in the stacking sequences over this compositional range(Fig. 11).

Another example of local phase re-equilibration during sulphidation isseen in the formation of jonassonite together with Bi-sulphotellurides inpatches containing symplectites of gold and bismuth and relictmaldonite(Fig. 3a). Such complex assemblages can also be derived from simplerassociations corresponding to binary eutectics such as bismuth+maldonite (241 °C), following reaction of types (2a) and (2b), or toternary eutectics of types (3a) and (3b):

Au2BiðmaldoniteÞ þ 13Biþ 2H2TeO3 þ 9SO2�4 þ 18H

þ

¼ 2AuBi5S4 ðjonassoniteÞ þ Bi4Te2Sðjose0 ite�BÞ þ 11H2Oþ 15:5 O2 ð2aÞ

Au2BiðmaldoniteÞ þ 13Biþ 2H2Teþ 9H2S¼ 2AuBi5S4 ðjonassoniteÞ þ Bi4Te2Sðjose0 ite�BÞ þ 11H2 ð2bÞ

Even though tellurium has three oxidation states which haverelevant stability fields in the loga O2 range of interest (Te4+, Te0 andTe2−), we use only Te4+ and Te2− in reactions (2a) and (2b), to repre-sent high- and low-sulphidation cases, respectively.

Au2BiðmaldoniteÞ þ 2Bi7Te3 ðhedleyiteÞ þ 10Biþ 13SO2�4 þ 26H

þ

¼ 2AuBi5S4 ðjonassoniteÞ þ 2Bi4Te2Sðjose0 ite�BÞ þ Bi4TeS2 ðjose0 ite�AÞþBi3TeSþ 13H2Oþ 19:5 O2 ð3aÞ

Au2BiðmaldoniteÞ þ 2Bi7Te3 ðhedleyiteÞ þ 10Biþ 13H2S¼ 2AuBi5S4 ðjonassoniteÞ þ 2Bi4Te2Sðjose0 ite�BÞ

þ Bi4TeS2 ðjose0 ite�AÞ þ Bi3TeSþ 13H2ð3bÞ

Considering the observed dominance of joséite-B together withjonassonite, reactions (3a) and (3b) are more plausible since they donot require Te input from the fluid. Such formation of the more complexassemblages fromsimpler ones is also supported by the textural evidence.

The widespread presence of gold–bismuth symplectites at Maldonis a third example of local re-equilibration of phases after their initialcrystallisation. The simplest hypothesis is that they represent abreakdown product of maldonite during cooling below 113 °C, assuggested by the Au–Bi phase diagram (e.g. Okamoto and Massalski,1983). Whereas the presence of maldonite in the same areas as thesymplectites (Fig. 4a) places a question mark on this hypothesis,its presence in blebs several mm away in the same sample (Fig. 4c–e)can be attributed to metastability.

Fig. 11. Phase diagram for the Bi–Bi2Te3 sub-system, redrawn after Okamoto and Tanner(1990), representative for tellurides in the tetradymite group. This shows the continuouschange in composition, resembling a solid solution without a miscibility gap, along boththe solvus and solidus curves. The typesof structuralmodules for intervals betweenphaseswith integer k values are shown. If an assemblage of bismuth+hedleyite evolves towardsa Bi8Te5 polysome, and further towards Bi4Te3 (arrow), the stacking sequences wouldcontain modules with stepwise decreasing k towards Te-rich compositions (from 3 to 2betweenBi7Te3 and Bi4Te3). This is reflected in the fact that a common type ofmodule (9′)is preserved in the stacking sequences over this compositional range. Bi8Te3 (k=4) andBi8Te5 were not shown by Okamoto and Tanner (1990) but are added here to show theend-member for the interval with 9′ and 11′ modules, and the unnamed phase reportedhere, respectively. Even though the diagram reflects S- and Se-free species, the S-bearingspecies as in the present study,will be isostructuralwith the corresponding tellurideswithsame Bi:chalcogen ratio. Compositions given with asterisks represent named minerals(Bi7Te3—hedleyite, Bi4Te3—pilsenite, BiTe—tsumoite, Bi2Te3—tellurobismuthite). E1 repre-sents the eutectic between bismuth and hedleyite at 266 °C.


Partial melting of the maldonite+bismuth assemblage at eutectictemperatures (241 °C), or slightly below if we consider the presenceof impurities (e.g. Ag in gold, Sb in bismuth, Te in maldonite; Tables 2and 3), may also explain formation of marginal domains ofsymplectites. Some of the resultant Au–Bi melts may have beendriven away, resulting in the blebs discussed above. An alternativemechanism for symplectite formation is to consider pseudomorphicreplacement of parent maldonite during disequilibrium reaction withS-bearing fluids. Pseudomorphism and symplectite formation areboth key features of replacement coupling dissolution with repreci-pitation reaction rates (CDRR; e.g. Putnis, 2002). This would alsoexplain the different morphology of the symplectite boundariesagainst bismuth and maldonite (Fig. 4b), indicating advance of thereaction towards the inner part of the maldonite. Such morphologyhints at CDRR with molar excess as interpreted for replacement ofnagyágite by symplectites of galena and altaite (Sung et al., 2007;Ciobanu et al., 2008). CDRR can be used to transform other goldminerals into metallic gold, as shown by experiments on calaverite(Zhao et al., 2009).

An episode of sulphidation destabilising the maldonite could alsobe associated with the presence of sulphotellurides in some of thesymplectite-bearing patches (Fig. 4a). Illustrating this is reaction of thetypes (4a) and (4b):

5Au2BiðmaldoniteÞ þ 2H2TeO3 þ SO2�4 þ 2H

þ

¼ 10Auþ Biþ Bi4Te2Sðjose0 ite�BÞ þ 3H2Oþ 3:5 O2 ð4aÞ

5Au2BiðmaldoniteÞ þ 2H2Teþ H2S¼ 10Auþ Biþ Bi4Te2Sðjose0 ite�BÞ þ 3H2 ð4bÞ

A distinct subsequent overprint of the Au–Bi–Te–S assemblages byS-rich fluids is seen in the replacement of these assemblages bybismuthinite. This appears tied to a pronounced chloritisation event,even though it is bismuthinite that is always observed at the directreplacement fronts (Fig. 3e). The same event is recognised in pressuregashes and along fractures that crosscut the bleb-bearing fields(Fig. 3c). Replacement is most intense where gold-bearing mineralsare present (either maldonite or jonassonite). Textural evidenceshows that reaction between bismuthinite and jonassonite[reactions (5a) and (5b)] is reversible and is locally buffered bynative bismuth (Fig. 4d).

2AuBi5S4 ðjonassoniteÞ þ 7SO2�4 þ 14H

þ

¼ 5Bi2S3 ðbismuthiniteÞ þ 2Auþ 7H2Oþ 10:5 O2 ð5aÞ

2AuBi5S4ðjonassoniteÞ þ 7H2S ¼ 5Bi2S3ðbismuthiniteÞ þ 2Auþ 7H2 ð5bÞ

5.3. Thermodynamic modelling

None of the reactions (1a,1b) to (5a,5b) above can be modelledbecause of the lack of thermodynamic data for the sulphotelluride speciesor for jonassonite. These reactions are, however, illustrative of thepossiblerelationshipsbetweendifferentminerals in theassociations—inparticular,the formation of several telluride species from simpler assemblages.Whereas changes introduced by reactions (1a,1b) to (4a,4b) areassociated with modifications within the patches, reactions (5a) and(5b) define a different style of texture, involvingmarginal corrosion of thepatches. The latter is also true for replacement of maldonite bybismuthinite and gold, such as seen in Fig. 12a, which can be modelledthermodynamically using the modules Rxn and Act2 from The Geoche-mists Workbench (GWB) package 6.0 (Rockware Inc.; http://www.rockware.com).

Reactions (1a,1b) to (4a,4b) can be attributed to interaction withorogenic fluids during one or the other stages of deformation, inwhich cases the low-sulphidation type of reactions (reactions 1b to4b, involving H2S and H2Te) are more likely, given the typical near-neutral type of fluids involved (e.g. Mikucki, 1998). Reactions (5a) and(5b) are, however, more likely to involve magmatic (granitic) fluids,in which case the high-sulphidation type of reaction (involving SO4

2−)should be considered (e.g. Hedenquist, 1987). Considering SO4

2− asthe sulphur species for replacement of maldonite by bismuthinite andgold, we can write reaction (6):

6Hþ þ 2Au2BiðmaldoniteÞ þ 3SO

2�4

¼ 4Auþ 4:5 O2ðaqÞ þ 3H2Oþ 1Bi2S3 ðbismuthiniteÞ ð6Þ

We obtain a polynomial fit for the reaction with the equilibriumequation: log K=6×pH+4.5×loga[O2(aq)]. The system is in equilibri-um at 258 °C for loga O2(aq)=−32 and a pH of 5.7 (near-neutral at thistemperature); log K=−109.8. Products are favoured above this

http://www.rockware.com

http://www.rockware.com

Fig. 12. (a) Reflected-light photomicrograph in air showing replacement of maldonite by gold and bismuthinite for which thermodynamic modelling has been applied. (b–d) loga O2 vs. pHdiagrams at 258 °C, showing stability fields for (b)maldonite and gold, (c) bismuthinite and gold and (d) pyrite, hematite, magnetite and pyrrhotite, and for related aqueous species. In (b) thestabilityfields of bismuthinite (from c) and pyrite (fromd) are superimposed as blue and green lines, respectively. In b, the grey rectangle shows the range of logaO2 values for the replacement(reaction6)atpH=5.7. Conditions: logaSO4

2−=0, logaH2O=0and logaCl−=−1. In (b) and(c) logaAu+=−3, logaBi(OH)3(aq)=−5, in (d), logaFe2+=0.Anarbitrarypressureof2000 baris taken. Thermodynamic data for maldonite and bismuthinite are as used by Tooth et al. (2008). The reader is referred to the data depository to that paper for details and data sources.Data for gold and iron minerals are from the GWB default database (thermo.com.V8.R6), oct94.


temperature. Reducing pH to more acidic values would drop thetemperature (e.g. 233 °C at pH=4). Even though the speciation ofsulphur used reflects ahigh-sulphidation type offluid, the fact that thereare no index minerals for a high-sulphidation assemblage at Maldonsuggests that a near-neutral pH is more realistic.

Stability fields of Au–Bi–S phases and aqueous species are shown onloga O2(aq) vs. pH diagrams constructed at this temperature (Fig. 12b, c),using the following concentrations for aqueous components of thesystem: loga Au+=−3, loga Bi(OH)3(aq)=−5, loga SO4

2−=0, logaH2O=0 (see also caption to Fig. 12). There is a coincidence of the stabilityboundaries between maldonite and bismuthinite on the one hand, and

maldonite and gold on the other, at pH=5.7 for loga O2(aq)∼−32. Thetemperature of 258 °C is reasonable for the retrogradefluids in the contactaureole as discussed above.

At the same temperature and identical values for common compo-nents in the systems Au–Bi–S and Fe–O–S, we are close to the pyrite–hematite buffer [reaction (7)].

2FeS2ðpyriteÞ þ 4H2Oþ 7:5 O2ðaqÞ ¼ 8Hþ þ 4SO

2�4 þ 1Fe2O3ðhematiteÞ ð7Þ

We obtain a polynomial fit for the reaction with the equilibriumequation: log K=−4×pH−3.75×loga [O2(aq)]. The system is in


equilibrium at 258 °C for loga O2(aq)=−33.9 and a pH of 5.7; logK=208.7. Reactants are favoured above this temperature.

The pyrite–hematite buffer on the loga O2(aq) vs. pH diagram for thesystem Fe–O–S drawn at this temperature (Fig. 12d) shows logaO2(aq)=−33.7 at pH=5.7. Thepyritefieldbroadlyoverlapswith thebismuthinitefield (Fig. 12b).

At higher temperatures (e.g. at 300 °C), both bismuthinite and pyritefields decrease in size, but the latter decrease is more pronounced.Secondly, the maldonite–bismuthinite and maldonite–gold boundariesare both shifted towards higher loga O2(aq) (both∼−29 at 300 °C). Thesame shift towards higher loga O2(aq) is shown by the pyrite–hematitebuffer at pH=5.7 (∼−30 at 300 °C).

The loga O2(aq) conditions found for the replacement of maldoniteby bismuthinite and gold are thus more than ten orders of magnitudehigher than conditionsof pyrrhotite stability (pyrrhotite–pyrite buffer isat loga O2(aq)∼−46; Fig. 12d). Although no Fe-minerals are presentin the studied samples, replacement of maldonite by bismuthinite andgold cannot be associated with the latter buffer at which re-equilibration between löllingite, arsenopyrite and pyrrhotite takesplace following granite emplacement (Hughes et al., 1997).

Based on the textures andmodelling above, three stages—all involvinggold minerals—can be substantiated: (1) bismuth+maldonite±hedle-yite; (2) Bi-sulphotellurides+jonassonite, and decomposition of mal-donite (gold+bismuth); and (3) bismuthinite+gold fromdecomposition of maldonite (modelled here) or jonassonite. Formationof chlorite, although part of stage (3), as indicated by textures, may occurtowards the end of the stage. Fluids may have suffered subtle, localisedchanges, as indicated by observation of the reverse reaction (bismuthiniteand gold→jonassonite).

6. Discussion: how many Au events?

The apparent scarcity of maldonite in the Maldon goldfield (Birchand Ciobanu, 2009, and references therein), especially further awayfrom the contact to the granite, could be explained by replacementfollowing the two types of reactions discussed above. They lead toformation of: (i) symplectites of gold and bismuth [reactions (4a) and(4b)]; and (ii) bismuthinite+gold [reaction (6)]. Whereas thebreakdown to symplectites will still preserve a reduced associationin which bismuth, sulphotellurides and jonassonite are stable, andwhich would concord with the unusually high pyrrhotite:pyrite ratiomentioned for the ores (e.g. Ebsworth et al., 1998), the replacementby bismuthinite and gold can only take place at more oxidisingconditions due to a sulphidation reaction. Among the sulphotellur-ides, only phases like tetradymite (mentioned from Maldon byHughes et al., 1997, but not seen by us) would be stable. The apparentgold enrichment in the ore (presence of native gold), resulting fromeither type of maldonite replacement, does not require any addition ofgold from the fluids. The second reaction should be enhanced furtheraway from the granite contact since this is where pyrite rather thanpyrrhotite is reported to dominate the ores. This is also concordantwith a down-temperature increase in both the pyrite and bismuthi-nite stability fields. Thus, considering that the chloritisation accom-panying replacement (ii) is clearly associated with graniteemplacement, we conclude that the retrograde fluids in the contactaureole did not necessarily introduce gold, even though they causedthe formation of abundant native gold.

Unlike the retrograde fluids involved in reaction (6), the graniteaffiliation of the S-bearing fluids invoked for reactions (1a,1b) to(5a,5b), is a matter of debate. Sulphidation of initial Au–Bi±Teassociations could also be produced during the multi-stage orogenicdeformation recorded in the reefs (e.g. Ebsworth and Krowkowski deVickerod, 1998; Ebsworth et al., 1998), with no additional gold added.In this case, reactions (3a,3b) and (4a,4b) acting on a precursormaldonitewould also have modified the mineralogy of the ores. Together withformation of the multi-component sulphotelluride associations, this

further emphasizes aspects relating to a grain-scale remobilization ofearlier and generally simpler assemblages. Although the gold contentmeasuredwithin the sulphotellurides fromMaldon by laser-ablation ICP-MS (Ciobanu et al., 2009a) is no more than 1 ppm, it nevertheless isconcordantwith grain-scale remobilization of gold during reactions of thetypes above that involve sulphotellurides and gold minerals.

We suggest that initially S-poor assemblages crystallised fromreducedAu–Bi–Te–(S) melts that could have been partitioned from fluids duringone of themain stages of deformation that predated granite emplacement(e.g. D4; see Section 2 above). Such a mechanism would lead directly toformation of high-grade ore. Leaching of pre-existing Au–Bi±Te patchesby S-bearing fluids may also account for precipitation of melt droplets,such as those discussed here, during overprinting. The latter event(s),whether during subsequent deformation or following granite emplace-ment has further refined the ore, naturally increasing the proportion ofnative gold relative to the initial maldonite, which in our scenario mayhave initially been the dominant gold mineral.

In conclusion, we argue that the mineralisation at Maldon was theproduct of (i) a single main event involving gold-rich fluids, and (ii) atleast two sulphidation events that produced local-scale reworking andremobilisation of the initial ore. Such a scenario is concordant with somepreviouslypublished interpretationsof oregenesis atMaldon(e.g.Hugheset al., 1996).

The example of Maldon is representative for many metamorphicterranes where intrusion emplacement pre- or post-dates an orogenicevent (cf Bierlein et al., 2001). Associations of the type discussed here(Au–Bi–Te–S), including diverse assemblages from the tetradymitegroup, are also common features in such cases. Few, however, preservethe complexity of Maldon, especially in terms of gold phases, as seen inthe samples selected for the present study. Our approach is suited fordirect application to any type of gold deposit resulting froma protractedgeological history, but may carry an economic significance for those inwhich Bi-minerals are more than minor components in the ores, e.g.intrusion-related Au–Bi deposits (Baker et al., 2005).

Acknowledgements

This work was supported via an ARC Discovery fellowshipDP0560001 to CLC and DP0880884 to AP. This is a contribution toInternational Geoscience Project 486 ‘Gold-telluride deposits’. Wegratefully acknowledge the support of the Australian Microscopy andMicroanalysis Research Facility (AMMRF) for use of the FEI HeliosnanoLab DualBeam FIB/SEM system at Adelaide Microscopy; LeonardGreen is thanked for his assistance. The Minerals, Metals and SolutionsGroup, South Australian Museum, is kindly thanked for access to theirupdated thermodynamic database. We thank Yves Moëlo and ananonymous reviewer for their helpful comments that greatly assistedimprovement of the manuscript. This paper is contribution number 62to TRaX.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.lithos.2009.12.004.

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Documents

Petrogenetic significance of Au–Bi–Te–S associations: The example of Maldon, Central Victorian gold province, Australia